Polybutadiene rubbers having glass transition temperatures of about -95.degree. C. can be made by anionic polymerization with alkyl lithium initiators. Such polybutadiene rubbers generally have a narrow molecular weight distribution and a high cis-isomer content. The processability of such rubbers is not optimal by virtue of their narrow molecular weight distribution. Additionally, tire treadwear characteristics could be improved by increasing the level of trans-isomer content in the rubber.
Copolymers of .alpha.-methylstyrene and 1,3-butadiene are normally made by emulsion polymerization. Such emulsion polymerizations generally result in a high degree of branching in the .alpha.-methylstyrene/butadiene rubber which leads to a high level of hysteresis. This is undesirable in tire tread rubber compounds because high hysteresis causes poor rolling resistance characteristics.
Alpha-methylstyrene does not usually homopolymerize or copolymerize with conjugated diene monomers at temperatures above about 60.degree. C. (the ceiling temperature of .alpha.-methylstyrene polymerization). This, compounded by the fact that conjugated diolefin monomers normally polymerize at a much faster rate than .alpha.-methylstyrene, has precluded the synthesis of such copolymers by solution polymerization on a commercial basis.
Higher temperatures generally promote a faster rate of polymerization. Accordingly, it is desirable to utilize moderately high temperatures in commercial polymerizations in order to maximize throughputs. However, it has traditionally been virtually impossible to copolymerize .alpha.-methyl styrene with conjugated diolefin monomers at temperatures above about 60.degree. C. At temperatures below 60.degree. C. polymerizations have not proven to be commercially viable due to low polymerization rates and low conversion levels.